1. Field of the Invention
The present invention relates to an ink for ink-jet recording to be used for an ink-jet recording apparatus.
2. Description of the Related Art
In the ink-jet recording system, ink droplets are formed by means of the ink discharge method including, for example, the electrostatic attraction method, the method in which mechanical vibration or displacement is applied to the ink by using a piezoelectric element or the like, and the method in which bubbles are generated by heating the ink to utilize the pressure generated thereby. All or a part of the ink droplets are adhered to a recording objective material such as paper to perform the recording. Those usable as the ink for ink-jet recording to be used for the ink-jet recording system as described above include those which are obtained by dissolving or dispersing a variety of water-soluble dyes or pigments in water or a liquid medium composed of water and a water-soluble organic solvent. In particular, the ink for ink-jet recording, which is based on the use of the pigment, is excellent, for example, in the water resistance and the light resistance as compared with the dye ink.
It is required for the ink for ink-jet recording based on the use of the pigment as described above, for example, that the minute discharge nozzle of the ink-jet printer is not clogged; the fixation performance (abrasion resistance, finger touch resistance) of printed matters is improved; the drying performance after the printing is improved; and the storage performance is improved so that the pigment particles are not coagulated or sedimented and no solid matter other than the pigment is deposited even the ink is stored for a long period of time at a high temperature or at a low temperature.
In order to perform the ink-jet recording in a normal discharge state with an ink-jet printer when the ink cartridge is installed for the first time and when the ink cartridge is exchanged, it is necessary that the ink for ink-jet recording is introduced into the ink flow passage without allowing any bubble to remain therein, and it is necessary that any bubble remaining in the ink flow passage is completely removed. For this purpose, the forcible ink-sucking/discharging operation for removing the bubbles, which is called “purge”, is usually performed in the ink-jet printer. However, in general, it is difficult to obtain the normal discharge state by means of only the effect of the purge operation as described above. It is necessary that the ink for ink-jet recording itself has excellent recovery performance upon introduction into the recording head.
In view of the above, a method is widely used, in which an acrylic polymer is added into the ink for ink-jet recording. Accordingly, it is possible to improve the recovery performance upon introduction into the recording head and the fixation performance of printed matters. For example, Japanese Patent Application Laid-open No. 10-120955 discloses an ink for ink-jet recording obtained by adding a self-dispersing copolymer resin composed of stearyl (meth)acrylate, (meth)acrylic acid, styrene-based monomer, and benzyl (meth)acrylate-based monomer. Japanese Patent Application Laid-open No. 2000-169769 discloses an ink for ink-jet recording obtained by dispersing or dissolving a self-dispersing type pigment and a copolymer composed of acrylic acid and maleic acid or anhydride thereof in an aqueous medium.
However, when the acrylic polymer as described above is added in such an amount that the effect is obtained as described above, then the discharge from the recording head becomes unstable, and the straight travel stability of ink droplets is deteriorated immediately after the start of the printing operation. A problem has arisen such that all of the fixation performance of printed matters, the recovery performance upon introduction into the recording head, and the straight travel stability of ink droplets during the discharge cannot be satisfied sufficiently by adjusting only the amount of addition of the acrylic polymer.